THE PHYTOPLANKTON SUCCESSION IN THE LOWER PART OF HYPERTROPHIC NAKDONG RIVER (MULGUM), SOUTH-KOREA

Investigations were carried out to determine the mechanism of phytopla nkton succession in the lower part of Nakdong River. Intensive monitor ing was conducted from April 1993 to April 1995 at weekly or biweekly intervals. This river is the main source of drinking water for more th an 8 million residents living in Pusan and the Southeastern region of Korea, and it is also important for industrial purposes. Due to the ra pid urbanization and industrialization, this river has become hypertro phic and it is heavily regulated (mean chi. a, 79 +/- 232 mu g 1(-1) n =76; mean TN, 3.3 +/- 2.0 mg 1(-1) n=30; mean TP, 165 +/- 108 mu g 1(- 1) n=45). Bacillariophyceae were dominant year-round in 1993, with spr ing and late fall peaks. Dominant communities in 1994 were Bacillariop hyceae in March and April, Chlorophyceae and Cryptophyceae in May, Cya nobacteria in July and August, Chlorophyceae and Cryptophyceae in Octo ber, and Bacillariophyceae in December. As drought persisted through t he summer of 1994, elevated water temperature lover 30 degrees C) poss ibly triggered Cyanobacteria bloom (mainly Microcystis aeruginosa, max imum cell density, 1.6 x 10(7) cells ml(-1)). The most common diatom, Stephanodiscus hantzschii, repeatedly dominated from late fall to next spring (mean and maximum cell density, 7.5 x 10(4), 1.2 x 10(5) cells ml(-1), respectively). This small centric diatom was favored by the l ow discharge and the cold water (water temperature, 3-10 degrees C). A fter the diatom bloom, this community shifted to peaks of colonial Chl orophyceae and motile Cryptophyceae, owing to the high rate of zooplan kton grazing activity and increased water temperature. Overall, the ph ytoplankton periodicity was primarily governed by the hydrologic regim e (discharge). Changes in silica concentration, water temperature and high zooplankton density might have played an important role in phytop lankton dynamics during the non-flooding periods. Compared to other la rge rivers, strong Microcystis bloom events in summer and Stephanodisc us bloom events in winter were noticeable in the dry year. The pattern s of phytoplankton succession observed in this study may have a signif icance since most of the large rivers in Far Eastern Asian countries a re subjected to eutrophication and regulation of discharge.